1. Technical Field of the Invention
The present invention relates to a driving apparatus of a resonance type linear motor for driving a movable element supported by an elastic member using a reciprocal motor (linear motor).
2. Related Art
Hitherto, as the device using a resonance type linear motor, for example, a linear compressor for air conditioner is known. The linear compressor is designed to generate compressed gas, by making use of a mechanical elastic member or elasticity of compressed gas and linear motor.
Referring to FIG. 13, a structure of linear compressor using a spring as elastic member is detailed. A piston 61 is movably supported on a cylinder 60 along its axial direction. A magnet 62 is fixed to the piston 61. A stator coil 64 buried in an outer yoke 63 is disposed at a position facing the magnet 62. A compression chamber 65 formed of the cylinder 60 and piston 61 is connected to a suction pipe 66 and a discharge pipe 67. The suction pipe 66 has a suction valve 68, and the discharge pipe 67 has a discharge valve 69. The piston 61 is elastically supported by a resonant spring 70. In FIG. 13, providing a sinusoidal current flowing in a linear motor section 71 including the outer yoke 63, stator coil 64 and magnet 62 by way of a motor driver (not shown) moves the piston 61 reciprocally in its axial direction. Thus thee refrigerant is sucked and compressed in the compression chamber 65.
To operate such a linear compressor at high efficiency, it is required to drive it by matching the resonant frequency of the linear compressor with the frequency of the input current into the linear motor section 71. The resonant frequency of the linear compressor is determined, in the case of a structure including an elastic member, based on the elastic member as a mechanical member (machine spring), the elasticity generated by compressed gas (gas spring), and the mass of the piston 61. However, the elasticity caused by compressed gas varies significantly depending on load fluctuations, and thus the resonant frequency of the linear compressor cannot be determined uniquely. Accordingly, making use of the phenomenon that a resonant state takes place when the input current and the piston speed are equal in phase, it has been proposed to calculate the varying resonant frequency (Japanese Laid-open Patent Publication No. 10-26083).
An example of such a method is explained briefly by referring to a flowchart shown in FIG. 14. At step S20, a sinusoidal current command value Iref to be entered in the linear compressor is created from a driving frequency f. At step S21, a present speed Vnow of the piston 61 is determined from the position information of the piston 61 transmitted from a position sensor provided in the linear compressor. At step S22, a phase difference of Iref and Vnow is determined, and the process goes to step S23 when Iref is advanced, to step S24 when both phases are equal, or step S25 when Iref is delayed. At step S23, since the present driving frequency is lower than the resonant frequency, the driving frequency f is increased and the process returns to step S20. At step S24, since the present driving frequency is equal to the resonant frequency, the driving frequency f is not changed and the process returns to step S20. At step S25, since the present driving frequency is higher than the resonant frequency, the driving frequency f is decreased and the process returns to step S20. In this way, the driving frequency is controlled to coincide with the resonant frequency, by using the position information of the piston 61 obtained by the position sensor.
However, such a control method of the resonant frequency has the following problems.
Firstly, in the process of the driving frequency nearing the resonant frequency, the amplitude of the piston (movable element) increases largely, and in a compressor of a large resonance factor, collision of piston and cylinder head may occur, or the refrigerating capacity of the compressor may fluctuate significantly. In particular, in an environment of vibration applied from outside of a vehicle or the like, the amplitude of the piston resonates at its resonant frequency, and such problem occurs.
Secondly, in this method, displacement of the piston in the cylinder must be measured, and therefore a displacement sensor must be incorporated in the linear compressor, and the volume and weight of the compressor are increased by the portion of the sensor. Still more, to calculate the speed by differentiating the signal from the displacement sensor, or to calculate the phase difference of speed and current, a relative complicated control device such as microcomputer or MPU (microprocessing unit) is needed, and thus the cost is increased.
Thirdly, when using the displacement sensor, the displacement sensor must be sealed in a shell of the linear compressor. Thus it is required to guarantee the operation reliability of the displacement sensor in severe working conditions including temperature, pressure, and refrigerant-resistance.